The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well-known (Carlson, U.S. Pat. No. 2,297,691). The best known of the commercial processes, more commonly known as xerography, forms a latent electrostatic image on the surface of an imaging layer by uniformly electrostatically charging the surface in the dark, and then exposing the charged surface to a light and shadow image. The light-struck areas of the imaging layer are thus made substantially more charge-conductive and the electrostatic charge is selectively dissipated in such areas. After light exposure, the latent electrostatic image remaining on the imaging surface (i.e. a positive electrostatic image) is made visible by contacting with finely divided colored or black electroscopic material, known in the art as "toner". Toner is principally attracted to those areas on the image bearing surface which retain the original electrostatic charge and thereby form a visible positive image.
In structure, the conventional xerographic plate normally has a photoconductive insulating layer overlaying the conductive base or substrate and frequently an inteface or charge blocking layer between the two.
The photoconductive layer may comprise a number of materials known in the art. For example, selenium-containing photoconductive material such as vitreous selenium, or selenium modified with varying amounts of arsenic are found very useful in commercial xerography. Generally speaking, the photoconductive layer should have a specific resistivity greater than about 10.sup.10 ohm-cm (preferably 10.sup.13 ohm-cm) in the absence of illumination. In addition, resistivity should drop at least several orders of magnitude in the presence of an activating energy source such as light. As practical matter, a photoconductor layer should support an electrical potential of at least about 100 volts in the absence of light or other actinic radiation, and may usefully vary in thickness from about 10 to 200 microns.
In addition to the above, photoconductive layers will also normally exhibit some reduction in potential or voltage leak, even in the absence of an activating light. This phenomenon, known as "dark decay", will vary somewhat with the amount of usage of the photoreceptor. The existence of this problem is well-known and has been controlled, where necessary, by incorporation of an interface or barrier layer such as a very thin dielectric film or layer between the substrate and the photoconductive insulating layer. U.S. Pat. No. 2,901,348 to Dessauer et al utilizes a layer of aluminum oxide in this manner. Also of interest are thin films of a blocking resin interface such as a polybenzimidazole, a polyester, a polyurethane, a polycarbonate, an epoxy resin, or mixtures thereof, (0.1 to 2 microns). With some limitations, such blocking interface layers can effectively function not only to permit a photoconductive layer to support a charge of relatively high field strength, and to substantially minimize dissipation (dark decay) in the absence of illumination, but also to aid in cementing the photoconductive layer to the substrate. When activated by illumination, however, the interface-photoconductor layer combination must still be sufficiently conductive to permit dissipation or discharge of a substantial portion of the applied charge through the photoconductive layer. The above criterian is particularly important when one attempts to utilize xerographic processes in modern automatic copiers operating at high speeds. Flexible photoreceptors in the form of belts are typical of such usage. There are, however, serious technical problems inherent in their use. For example, high speed automatic cycling conditions require very fast charge dissipation under light exposure and also demand very strong adhesion between the photoconductor, the interface layer (where present) and the flexible substrate. In this connection, it has been well demonstrated that flexing of a photoreceptor for an extended period will inevitably crack substrate-photoconductor interfaces and result in the flaking off or spalling of sections of the photoreceptor.
The above problems are particularly acute when the newer, more sensitive inorganic selenium photoconductor alloys such as arsenic-rich selenium alloys (ref. U.S. Pat. Nos. 2,822,300, 2,803,542 and 3,312,548) are utilized as photoconductors. Such materials are brittle and best applied by condensing the vaporized alloy onto a prepared interface-substrate under vacuum. The heat of condensation of such alloys, however, is substantial, and substrates (i.e. thin foils) as well as optional thin polymeric interface layers are temperature sensitive. Moreover, there is no commercial product or knowledge in the art which suggests a way of completely avoiding the cracking and spalling problem with a flexible photoreceptor.
It is an object of the present invention to obtain improved photoreceptors with photoconductive surfaces having substantially less surface defects such as pitting and also adhering well to the substrate and to interface layers.
It is also an object of the present invention to obtain flexible, particularly belt-type photoreceptors having improved light sensitivity, stability and durability.
It is a still further object of the present invention to efficiently utilize arsenic-rich selenium alloys as photoconductors in successful working combination with flexible metal substrates having different coefficients of expansion than the photoconductor layer.